Physiography of bed-load meandering streams: imbricated gravels in fine-grained overbank deposits

Abstract

In the drainage basins of the meandering Central Sudetic rivers, the Holocene valley-floor deposits consist of three divisions: coarse gravel at the bottom (channel-lag deposits), fine gravel and coarse sand (point bar deposits) and silty clay to sandy clayey silt at the top (overbank deposits known also as so-called „alluvial loams"). As a rule, the latter contain imbricated pebbles that may be scattered throughout the „loam” or grouped into streaks. Field investigation shave demonstrated that the gravels are attributable to occasional overbank flows connected with heavy floods. C-fabric measurements of such gravels may serve as a directional, however, only in a statistic sense. The scatter in C-axis dip directions of the gravels is considerable but in general similar to that typical of channel-lag gravels of the same meandering stream. Fabric measurements in both overbank- and channel-lag gravels tend to indicate local downcurrent directions which almost always are deviated from a local downvalley direction. The deviations are clearly attributable to each channel sinuosity, bank effects and processes of lateral accretion. However, these highly variable down-current directions give low-concentration cumulative fabric diagrams of a well-defined monoclinic symmetry. It has been also found that the axis of symmetry of such diagrams may precisely indicate a mean downvalley direction provided that a sufficiently large number of fabric data is available.